1. Field of the Invention
The present invention relates to a gas laser oscillator having a function for judging initiation of discharge in a discharge tube of the oscillator.
2. Description of the Related Art
In a gas laser unit of the prior art, in order to initiate discharge at the time of startup of the gas laser unit, gas pressure within a discharge tube is lowered considerably. However, starting with such a low gas pressure, it takes a long time to increase the gas pressure so as to obtain a desired laser output. For example, Japanese Unexamined Patent Publication (Kokai) No. 2-126690 discloses a gas laser control unit, intended to generate a trigger arc in a discharge tube, wherein an ignite pulse is generated for supplying high-frequency power to a discharge tube at predetermined intervals so as to obtain a voltage equal to or larger than at the time of initiation of discharge.
On the other hand, as a technique for detecting initiation of discharge, Japanese Unexamined Patent Publication (Kokai) No. 7-221378 discloses a technique for detecting the initiation of discharge based on transition of characteristics of an output current, and detecting abnormal laser gas.
In the invention of Japanese Unexamined Patent Publication (Kokai) No. 2-126690, if a pulse having a needlessly large peak value or width is superimposed at constant intervals when it is difficult to initiate discharge, a switching semiconductor of a laser power supply may be overloaded, and the laser power supply may be damaged. Further, Japanese Unexamined Patent Publication (Kokai) No. 2-126690 does not disclose a particular technique for judging the initiation of discharge. Therefore, when a plurality of pulses are superimposed, a monitored value (or detected value) of the voltage of the discharge is affected by the superimposed pulses, and the initiation of discharge cannot be correctly judged.
In the invention of Japanese Unexamined Patent Publication (Kokai) No. 7-221378, since it is usually difficult to initiate the discharge under the high gas pressure, excess voltage may be applied to the discharge tube and an excess current may flow in the laser power supply before the transition of the characteristics of the output current occurs, depending on change of the state (for example, a pressure, a flow rate and a composition) of the laser gas. As a result, the discharge tube and/or the laser power supply may be damaged. Further, the characteristic of the output current, measured by a DC power supply part in the laser power supply, is varied depending on a plurality of components, i.e., an RF power supply part, a matching unit and a discharge tube. Therefore, it is difficult to correctly judge the initiation of discharge only.